The amount of heavy metals contained in flue gases released from high temperature incinerators, such as demilitarization furnaces operated by the U.S. Government and its contractors, is monitored and controlled in accordance with EPA regulations. A typical air pollution control technique is to inject activated carbon or its modified form (activated carbon impregnated with various chemicals) as sorbents into the flue gas or the combustion chamber to capture the metal vapors. The carbon is then separated from the flue gas in a downstream filter system. Chen, S., Rostam-Abadi, M., and R. Chang. An Evaluation of Carbon-Based Processes for Combined Hg/SO2/Nox Removal from Coal Combustion Flue Gasses, Book of Abstracts, 216th ACS National Meeting, Boston, Aug. 23-27, 1998.
As an example, the U.S. Army operates and maintains deactivation furnaces for conventional and chemical munitions demilitarization operations. These furnaces are subject to the Hazardous Waste Combustor (HWC) National Emission Standards for Hazardous Air Pollutants (NESHAP). The HWC NESHAP has stringent standards for lead and mercury emissions among various other volatile and semi-volatile metals and toxic organic compounds. Existing incinerators are currently not to exceed 240 micrograms/dry standard cubic meter (μg/dscm) for lead and cadmium emissions. The current compliance standard is set at 24 μg/dscm for new incinerators. These metal emissions exist as vapors near the furnace and afterburner in the air handling system. Thus a desired technology is one that adsorbs these metals while they exist as vapors at the high temperature locations in the air handling system before they condense to form sub-micron particulates that are extremely difficult to capture by conventional filtration systems. Existing adsorbent technologies are not effective at high temperatures and even if they could be made more effective they would be too costly to employ in this application.
A number of studies exist on the removal of lead vapor from a gas stream. Yang et al. (2001) reported a method of reducing volatile lead emissions from waste incineration by high temperature capture of vapor phase metals before they condense into fine particles. Packed bed sorption experiments with calcinated kaolin at 973-1173° C. were conducted. Lead reacts with the sorbent to form water insoluble lead-mineral complexes. Increased bed temperature resulted in increased capture rates, but it had no effect on maximum uptake. Diffusional resistance developed in the interior of the porous kaolin particles. This resistance became limiting only after the conversion of lead-kaolin reached a value greater than 50%. Yang, H., Yun, J., Kang, M., Kim, J., and Y. Kang; Mechanism and Kinetics of Cadmium and Lead Capture by Calcined Kaoline at High Temperatures; Korean J. Chem. Eng., 18(4), 499-505, 2001.
Vapors of lead chloride were removed by adsorption on a bed of Al2O3 at 570-650° C. The concentration of lead was measured during the adsorption process by low-energy radiation. The rate of adsorption increased with the flow rate. The amount adsorbed at saturation depended on the internal surface area of the adsorbent as well as on the particle size. Aharoni, C., Neuman, M., and A. Notea; Ind. Eng. Chem., Process Des. Dev. 14(4), 417-421, 1975.
Wronkowski (1965) reported adsorption of tetraethyl lead on two kinds of activated carbons at 18° C. with partial pressure in the range 0.03-0.9 atmospheres. The amount adsorbed depended on the specific surface of the given carbon and on the structure of its pores. Wronkowski, C., Adsorption of Tetraethyl Lead Vapors on Activated Carbon, Gaz. Woda Tech. Sanit., 39(4), 131-132, 1965.
Uberoi and Shadman (1990) evaluated several sorbents for removal of lead compounds, mainly PbCl2. The sorbents were silica, alpha-alumina, and natural kaolinite, bauxite, emathlite, and lime. The experiments were carried out at 700° C. At this temperature PbCl2 chemically reacted with the sorbent producing both water soluble and insoluble compounds. The authors provided relative sorption capacity, with kaoline giving the best result. Uberoi, M. and M. Shadman; High-Temperature Adsorption of Lead Compounds on Solid Sorbents; AIChE J., 36, 307, 1990.
Wey and his coworkers studied the adsorption mechanisms of heavy metals, including lead, on silica sands using a fluidized bed system at the temperature range of 600 to 800° C. Within this temperature range chemical reactions rather than a physical adsorption are preferred. They noted that for lead, both chemical and physical adsorption mechanisms are all-important depending on the reacting environment. Saturation adsorption capacities of silica sand for lead were 16.08 mg/g at 600° C. and 12 mg/g at 800° C. Chen, X., Feng, X., Liu, J., Fryxell, G. E., and M. Gong. Mercury Separation and Immobilization Using Self-Assembled Monolayers on Mesoporous Supports (SAMMS), Sep. Sci. Technol., 34(6&7), 1121-1132, 1999. Wey, M.-Y., Hwang, J.-H., and J.-C. Chen; The Behavior of Heavy Metal Cr, Pb, and Cd During Waste Incineration in Fluidized Bed Under Various Chlorine Additives; J. of Chem. Eng. of Japan, 29(3), 494-500, 1996.